Members of the ErbB family of receptor tyrosine kinases are important mediators of cell growth, differentiation and survival. The receptor family includes four distinct members, including epidermal growth factor receptor (EGFR or ErbB1), HER2 (ErbB2 or p185neu), HER3 (ErbB3) and HER4 (ErbB4 or tyro2). Both homo- and heterodimers are formed by the four members of the EGFR family, with HER2 being the preferred and most potent dimerization partner for other ErbB receptors (Graus-Porta et al., Embo 3 1997; 16:1647-1655; Tao et al., J Cell Sci 2008; 121:3207-3217). HER2 has no known ligand, but can be activated via homodimerization when overexpressed, or by heterodimerization with other, ligand occupied ErbB receptors.
The HER2 gene (also known as HER2/neu and ErbB2 gene) is amplified in 20-30% of early-stage breast cancers, which makes it overexpress epidermal growth factor (EGF) receptors in the cell membrane (Bange, et al., Nature Medicine 7 (5): 548-552). Besides breast cancer, HER2 expression has also been associated with other human carcinoma types, including non-small cell lung cancer, ovarian cancer, gastric cancer, prostate cancer, bladder cancer, colon cancer, esophageal cancer and squamous cell carcinoma of the head & neck (Garcia de Palazzo et al., Int J Biol Markers 1993; 8:233-239; Ross et al., Oncologist 2003; 8:307-325; Osman et al., J Urol 2005; 174:2174-2177; Kapitanovic et al., Gastroenterology 1997; 112:1103-1113; Turken et al., Neoplasma 2003; 50:257-261; and Oshima et al., Int J Biol Markers 2001; 16:250-254).
Trastuzumab (Herceptin®) is a recombinant, humanized monoclonal antibody directed against domain IV of the HER2 protein, thereby blocking ligand-independent HER2 homodimerization, and to a lesser extend heterodimerization of HER2 with other family members in cells with high HER2 overexpression (Cho et al., Nature 2003; 421:756-760 and Wehrman et al., Proc Natl Acad Sci USA 2006; 103:19063-19068). Herceptin® has been approved both for first-line and adjuvant treatment of HER2 overexpressing metastatic breast cancer, either in combination with chemotherapy, or as a single agent following one or more chemotherapy regimens. Trastuzumab has been found to be effective only in 20-50% of HER2 overexpressing breast tumor patients and many of the initial responders show relapse after a few months (Dinh et al., Clin Adv Hematol Oncol 2007; 5:707-717).
Pertuzumab (Omnitar/Perjeta™ also called 2C4) is another humanized monoclonal antibody directed against domain II of the HER2 protein, resulting in inhibition of ligand-induced heterodimerization (i.e., HER2 dimerizing with another member of the ErbB family to which a ligand has bound); a mechanism reported to not strictly require high HER2 expression levels (Franklin et al., Cancer Cell 2004; 5:317-328.). Pertuzumab is approved for the treatment of HER2-positive metastatic breast cancer, in combination with trastuzumab and docetaxel.
A HER2 antibody drug conjugate (ADC), Trastuzumab emtansine (ado-trastuzumab emtansine, Kadcyla®) is an antibody-drug conjugate consisting of the monoclonal antibody trastuzumab (Herceptin) linked to the cytotoxic agent mertansine (DM1). Kadcyla as a single agent, has been approved for the treatment of patients with HER2-positive (HER2+), metastatic breast cancer (MBC) who previously received trastuzumab and a taxane, separately or in combination.
While many factors are involved in selecting a suitable antibody for HER2 targeted therapy, it is typically an advantage for an ADC approach if the HER2-antibody complex efficiently internalizes upon antibody binding. As compared to EGFR, however, internalization of HER2 is impaired.
The complex mechanisms regulating the function of HER2 warrant further research on new and optimized therapeutic strategies against this proto-oncogene.
Accordingly, there exists a need for therapies that target the biological activities of HER2.